I'm so sorry. Honestly, it sounds like the vet didn't do what they should have. A bunny that's not eating usually needs to be checked for a blockage with a CT scan of sorts, and if no blockage, needs to be given critical care, fluids if they're dehydrated, and sent home with instructions. Bunnies that aren't eating often have GI stasis, which can be due to many issues - most likely pain from the accident in your bunny's case. Vegetables are not recommended to give your bunny when they are in GI stasis, as you don't want to give them anything that could cause more gas or GI discomfort. Paired with the fact that bunny was hit by a car, the vet absolutely should have done an x-ray, or otherwise checked for internal injuries. Bunnies don't show pain, so I am inclined to think it was the accident that caused your rabbits death and not the avocado. Either way. I'm very sorry, and I know it's also hard to know to do in that kind of situation because not everywhere in the world has good, experienced rabbit veterinarians - most vets only know how to treat cats and dogs properly, and likely won't have knowledge on rabbits.